At present, miniaturization and high-frequency are the trend of development of power supplies. However, the increase of switching frequency will result in the significant losses in the switch device. This can not be resolved by the conventional buck converter, while a resonant converter can resolve this problem properly.
Taking the series resonant converter as an example, the series resonant DC/DC converter adopts resonant converting technique. As the resonant element operates at the sine resonant state, the voltage across the switch device is naturally cross-zero, thereby realizing zero-voltage turn-on, and very small loss of the power supply. This topology usually utilizes pulse frequency modulation (PFM) mode, stabilizing the output voltage by changing the operating frequency. FIG. 1 is the basic form of the half-bridge DC/DC series resonant converter (SRC). When the circuit is controlled in PFM mode, the two switch devices S1, S2 are complementary symmetrically driven, and are respectively turned at 50% of the switching cycle. This is a desirable value, and should be slightly smaller than 50% in consideration of setting the dead zone. The relationship between the gain of the output voltage of the power supply M and the operating frequency f is:
                    M        =                                            V              O                                      V              in                                =                      0.5                                          Q                S                            ⁢                                                                                    f                                          f                      r                                                        -                                                            f                      r                                        f                                                                                                                          (        1        )            Wherein Vo and Vin are, respectively, the output voltage and the input voltage, f is the operating frequency,
            f      r        =          1              2        ⁢        π        ⁢                              L            ⁢                                                  ⁢                          r              ·              C                        ⁢                                                  ⁢            r                                ,          ⁢            Q      S        =                  2        ⁢                                  ⁢        π        ⁢                                  ⁢                  f          r                ⁢                  L          r                ⁢                  P          o                            U        o        2              ,fr is the resonant frequency, Lr is the resonant inductance, Cr is the resonant capacitance, and Po is the output power.
As seen from equation (1), when the operating frequency f is larger than the resonant frequency fr, a higher operating frequency results in a lower voltage gain M. Likewise, when the operating frequency f is less than the resonant frequency fr, a lower operating frequency results in a lower voltage gain M. The curve of FIG. 2 demonstrates the relationship between the controlling frequency f and the output voltage Vo. As seen from FIG. 2, the output voltage is less stable under light load and non-load conditions. When the controlling frequency is greater than the resonant frequency fr, the output voltage of the series resonant topology decreases as the controlling frequency continues to increase. The output voltage tends to be smooth when the load decreases to a light load state. Therefore, the operating frequency needs to significantly increase in order to stabilize the voltage. However, a broad range of operating frequencies will present challenges in optimizing the magnetic device. Higher operating frequencies can increase the loss of the circuit. In addition, when the load is close to non-load, the output voltage may rise instead, thereby presenting challenges to control negative feedback. Accordingly, in the power supply industry, designers may adopt a fixed load at the output in order to stabilize the output voltage under light load and non-load conditions. However, this operation may increase non-load loss and decrease the efficiency of the power supply.
Pure pulse frequency control will result in an overly broad range of operating frequencies, even failure, and make optimization of the magnetic device difficult and cause large circuit losses. This may result in difficulty designing feedback control. Therefore, simple frequency modulated control does not provide the desired voltage stability at light load or non-load conditions.
The above-described conditions of frequency modulated control have been described in connection with a half bridge series resonant circuit. A full-bridge series resonant circuit is subject to the same conditions as that of the half-bridge series resonant circuit. In theory, there are similar problems in all frequency modulation control resonant circuits.